Very high speed commercial digital presses print variable data at rates of thousands of pages per minute. Typically, the receiver media on which the data is printed is in the form of a web that is transported past stationary printheads. During transport, the web has considerable inertia and cannot be readily subjected to rapid changes in speed. It is desirable to continuously transport the web of receiver media at a constant speed, or with relatively slow speed adjustments.
The continuous transport of the web of receiver media also necessitates a continuous supply of data in the form of printable image data. This image data must be buffered so as to be available when it is needed, since any delay would result in blank pages unless the web of receiver media is stopped. Considerable time is required to fill a buffer with the printable image data, since the print job is initially supplied in an image description format which must be converted to image bitmaps. That conversion is typically in the form of raster image processing and is performed by one or more downstream processors. The processing speed that defines the sustained output speed of each of the downstream processors is generally limited by the content of the input descriptions, since the time to raster image process the image description for an individual print page tends to be highly data dependent. It is possible for the image description for a single print page to require a substantial amount of time to be converted to the printable image data output. Another limitation that affects the raster image processing is the supply of input data to the downstream processors. Variable data supplied by a secondary source can be subject to limitations of communication bandwidth or the processing capability of a host computer. With many print jobs, portions of the print job are simultaneously raster image processed and buffered before they are delivered to final raster image processor for assembly into a print engine ready format.
In high speed printing, a continuing problem is reliably supplying data at a rate sufficiently matched to the print engine. One approach to solving this problem is pausing the transport of the receiver media through the media transport path until sufficient data is available. This approach can be problematic, particularly for receiver in the form of a web being transported at high transport speeds. Another approach is to restrict how a print job is input. While this can be workable, it greatly encumbers the flexibility to make any last minute changes in the image data.
Commonly-assigned U.S. Pat. No. 6,762,855 (Goldberg et al.) discloses a system that uses buffer management logic to adjust transport speed on a per-document basis. Control buffers accumulate slack time left over from raster image processing non-complex documents and then allocate that slack time to complex documents to optimize average raster image processing time with the speed of the print engine.
The rate at which print data must be buffered necessitates high read and write data rates with the buffer memory in a memory storage unit. To overcome the inherent data transfer rate limitation of a memory storage unit, one method for achieving such data rates has been to use a plurality of memory storage units that can be concurrently accessed. However, the increase in the number of memory storage units brings with it an increased risk that one of the memory storage units will fail, bringing with it a risk of a total printing system failure.
There remains a need to provide the high data rates required for storing and accessing image data for high speed printing without producing an undue risk of a printing system failure.